A projection display has been heretofore available which converts white light from a light source into linearly polarized light, decomposes the light into red (R), green (G), and blue (B) lights, enters the three color lights on their respective reflective liquid crystal display elements via polarization beam splitters, combines the color lights reflected from the elements into one by color combining optical system, and causes the combined light to enter projection optical system such that an image in full color is projected and displayed on a screen. In this display device, each reflective liquid crystal display element performs polarization modulation to vary the orientation of polarization such that each color light possesses different image information. The color lights which have been polarization-modulated are combined into one by crossed dichroic prisms for color combination. The emerging combined light is projected toward the screen through the projection optical system. An image is displayed on the screen.
In this projection display, a part of the combined light incident on the projection optical system is reflected from lenses or apertures within the projection optical system, thus producing noise light. The noise light passes through crossed dichroic prisms and deflection beam splitters and reenter the reflective liquid crystal display elements. The light is again reflected here and combined with the aforementioned combined light. The produced light is projected onto the screen as a ghost image, thus presenting a problem. It is known that this problem can be alleviated by techniques described, for example, in JP-A-9-251150 and JP-A-2004-29692.
In the technique described in JP-A-9-251150, a λ/4 wave plate is inserted between the projection optical system and the crossed dichroic prism assembly for color combination such that the orientation of polarization of the noise light reflectively returning from the projection optical system is rotated through 90°. Accordingly, the orientation of polarization of the noise light when it has returned again to the polarization beam splitters through the crossed dichroic prisms is s-polarized light that is produced by rotating normal, polarization-modulated light (assumed to be p-polarized light) through 90°. Therefore, the noise light is discarded off the optical path without reentering the reflective liquid crystal display elements via the polarization beam splitters.
In the technique described in JP-A-2004-29692, the λ/4 wave plate used in the technique of JP-A-9-251150 is employed. In addition, a dichroic mirror that transmits red light but reflects other color lights is disposed between the polarization beam splitter for red light and the pair of crossed dichroic mirrors. Another dichroic mirror that transmits blue light but reflects other color lights is disposed between the polarization beam splitter for blue light and the pair of crossed dichroic prisms. Noise light that is different in orientation of polarization is discarded by making use of the characteristics of polarization beam splitters as in JP-A-9-251150. In addition, noise light of green color traveling toward the reflective liquid crystal display elements for red and blue lights is discarded off the optical path by reflection off the dichroic mirrors.
However, it is known that the dichroic film used in crossed dichroic prisms shows spectral transmissive or spectral reflective characteristics which are not always uniform about p- and s-polarized light and hence has some degree of dependence on wavelength. Therefore, even if a λ/4 wave plate is used as described in JP-A-9-251150 1, and if noise light reflected from the projection optical system is made to reenter the crossed dichroic prisms, for example, as p- or s-polarized light by the λ/4 wave plate, it is unlikely that the dichroic film correctly spectrally decomposes the noise light and the color lights reenter their respective polarization beam splitters. Some of the p- or s-polarized light of other color light enter the polarization beam splitters. The result is that a part of noise light of other color light reenters the reflective liquid crystal display elements mounted for certain color lights. Ghost image is not suppressed sufficiently. This adversely affects the color reproducibility.
In this respect, if a wavelength selective dichroic mirror is used in combination to prevent other color lights exiting from the crossed dichroic prisms from entering polarization beam splitters mounted for certain color lights as known in JP-A-2004-29692, an improvement will be achieved. However, in order to discard unwanted color lights off the optical path, the dichroic mirror is mounted at an angle to the optical axis. Nonetheless, the spectral transmissive and reflective characteristics of the dichroic mirror have dependence on the incidence angle. Furthermore, normal polarization-modulated light exiting from the reflective liquid crystal display elements enter the dichroic mirror at various angles. This is a factor causing color nonuniformity in the projected image. Additionally, the dichroic mirror is made of a transparent base plate such as a glass plate on which a dichroic film of multilayer film configuration is formed and so the astigmatism, for example, in the projection optical system may be deteriorated depending on the thickness of the base plate.